Most cameras on the market generate 3-band image data, generally composed of the red, green and blue (“RGB”) bands. It is known for a camera to generate 3-band image data through the use of a single charge-coupled device (“CCD”). A CCD is a device which converts photons of light, such as those which are received through the lens of a camera, into electrical signals. CCDs used in cameras generally have a two dimensional (“2D”) array of pixels, with each pixel generating its own electrical signal corresponding to the incident light energy.
It is also known for multiple specialized cameras to generate 4-band image data. Such 4-band image data may be composed of RGB and near infra-red (“NIR”) bands. However, the multiple specialized cameras generating 4-band data have used more than one CCD in the process, such as in two separate cameras. For example, two cameras have been used, each with one CCD, along with a lens system and an appropriate optical filter. One camera may be configured as an RGB camera with a visible optical filter, while the second camera may be identical to the first camera except using an optical NIR filter in place of the visible optical filter. This allows NIR light to expose the CCD of the second camera while blocking the visible light. Both cameras record the imagery onto a recording device during in-flight operation. During post-flight processing, the two images (RGB and NIR) are combined to produce a 4-band image. Registration errors due to slight mis-alignments between the two cameras (and their respective CCDs) are inherently present for the two camera system. In addition, the use of multiple cameras and CCDs adds to the cost and complexity of the 4-band system.
There are many CCDs available on the market, including the Truesense Class 1 KAI-29050 CCD, manufactured by Truesense Imaging, Inc. (formerly Kodak). CCDs are described, for example, in U.S. Patent Publication No. US 2007/0024931 of Compton et al. The KAI-29050 CCD has pixels which are sensitive to panchromatic (“pan”), red, green and blue light, and is contained within industrial cameras such as those made by Imperx or Illunis, but such a single CCD has only been used to generate 3-band image data. It has not been known how to use only a single such CCD in the generation of 4-band data.
Certain filters are also known to be useful in connection with cameras, including a variety of off-the-shelf neutral density (“ND”) filters. ND filters equally attenuate wavelengths of light, at least within the visible spectrum, but, particularly with inexpensive ND filters, the amount of attenuation outside the visible spectrum can be different. For example, the Tiffen ND0.6 filter attenuates significantly less NIR light than visible light. But, again, it has not been known how to employ such filters to generate 4-band data using a single CCD.
Moreover, color calibration technology is known in connection with cameras. Such color calibration may make use of color calibration targets 90, such as a Macbeth® ColorChecker2 target (shown in FIG. 5). One use of this target has been for 3×3 (visible) calibration.
Most cameras use a CCD that employs the Kodak Bayer pattern (shown in FIG. 7). Some others use the TrueSense CFA pattern. The Kodak Bayer filter pattern employs three colors, arranged in a mosaic on a square grid of photosensors; the filters being 50% green, 25% red, and 25% blue. Various de-mosaicing software programs have been used to process imagery collected using the Bayer pattern to interpolate 3-band data, specifically: red, green and blue values for each pixel. Additional information about this pattern is available in Dr. Bayer's (of Kodak) U.S. Pat. No. 3,971,065.
TrueSense Imaging, Inc. introduced the TrueSense CFA pattern to improve color imaging at lower light levels because panchromatic masked pixels are more sensitive to incident light. The TrueSense CFA pattern (shown in FIG. 7) employs four colors, arranged in a mosaic on a square grid of photosensors; the filters being 50% panchromatic, 25% green, 12.5% red and 12.5% blue. Software has been created to de-mosaic images, such as that created by TrueSense Imaging, Inc. Such de-mosaicing software is described in U.S. Patent Publication Number 2007/0024879 A1 of Hamilton et al. However, it has not been known how to use such targets 90, patterns 80 and de-mosaicing software in the generation of 4-band data using a single CCD.
Accordingly, there exists a need for new and improved cameras, and methods of use for such cameras, that are capable of generating 4-band data using only a single CCD. In particular, there exists a need for such cameras 60 and methods that take advantage of targets 90, patterns 80, specialized filters (20;40), and custom de-mosaicing software to generate high-quality, radiometrically accurate 4-band image data. Such data is of particular use in the field of airborne photogrammetry, where visible spectrum data (such as RGB data) as well as NIR data is exploited. Cameras and methods useful for generating radiometrically accurate 4-band image data are also of particular value in the Geographical Information System (“GIS”) community. For example, RGB imagery can be used for visualization purposes and in addition, the NIR data coupled with the RGB data can be used in agricultural and forestry applications to obtain useful information on vegetation classification, health and stress.